Liquid crystal display (hereinafter referred to as LCD) devices that have been used generally have employed polarizing films since they have used linearly polarized light. Polarizing films are fabricated by adsorbing an iodine or dichroic dye to a polyvinyl alcohol (PVA) film and stretching the adsorbed film in a fixed direction.
However, the polarizing films manufactured as described in the above are not practical in that they themselves have a weak mechanical strength with respect to the direction of a transmission axis and the function of polarization is lowered significantly as they are contracted by heat or moisture. In order to complement this deficiency, polarizing films having a structure in which an adhesive has been adhered to between supports such as an acetic acid cellulose film, etc. have been developed.
Since the polarizing films using the above polyvinyl alcohol films adsorb light progressing in one direction but pass only the light vibrating in another direction producing a linearly polarized light, the efficiency of the polarizing films can not exceed 50% theoretically, which is the most significant factor for lowering the efficiency and brightness of LCDs.
It is possible to improve greatly disadvantages of the conventional polarizing films by. using reflective polarizing films additionally that are manufactured by using cholesteric liquid crystals. Cholesteric liquid crystals have a selective reflection characteristic. That is, the twisted direction of the spiral structure of liquid crystals and the oriented direction of circularly polarized light are consistent with each other, and cholesteric liquid crystals reflect only the circularly polarized light of which wavelength is the same as the spiral pitch of the liquid crystals. It is possible to manufacture polarizing films that can convert non-polarized light in a fixed wavelength band into a specific circularly polarized light by using the selective reflection characteristic.
In other words, if non-polarized light in which a left circularly polarized light component and a right circularly polarized light component are mixed equally is incident to a cholesteric liquid crystal film having a left- or right-handed spiral structure, the circularly polarized light of which direction is the same as the spiral direction is reflected, while the circularly polarized light of which direction is in the opposite direction to the spiral direction is transmitted. The transmitted circularly polarized light is converted to linearly polarized light after it passes thorough a ¼ λretardation film. The circularly polarized light reflected thus transmits the liquid crystal film since its polarization direction is changed if it is reflected again from a reflective plate. Accordingly, it is possible to improve brightness remarkably by using additionally polarizing films manufactured by using cholesteric liquid crystal films compared to the cases of using only the conventional absorption-type polarizing films that absorb 50% of light since there is no loss of light theoretically.
However, the backlight used for an LCD generates visible light (400-700 nm) that is in a region showing colors mainly. Therefore, the selective reflection wavelength region of a cholesteric liquid crystal film should cover the visible light region. If it fails to do, the light in a wavelength region that has not been utilized again after selective reflection transmits the polarizing film in a non-polarized state and brings about a problem of lowering the picture quality of the LCD.
Also, an LCD uses many kinds of functional films overlapped in the backlight unit, where brightness is lowered since light scattering phenomena occur at interfaces every time the light passes through each film. Among many kinds of LCD backlight films, prism-patterned films that can improve brightness by collecting the light generated from the light source to the front are used. Generally, prism films are manufactured in the coating method by forming prism patterns on the films. Two prism-patterned films are overlapped for them to be used, but they still have the problem of light scattering at interfaces as in the conventional methods.